This study will investigate if treatment with vernakalant hydrochloride (MK-6621) results in a greater proportion of patients with treatment-induced conversion of Atrial Fibrillation to sinus rhythm compared to placebo.

Amendment 4 of the protocol revised the planned enrollment to 123 participants and removed China as one of the countries participating in the study.

Inclusion Criteria:

• Abstinent or willing to use 2 acceptable methods of birth control
• Patient has an atrial arrhythmia with dysrhythmic symptoms
• Patient is receiving adequate anticoagulant therapy
• Patient has stable blood pressure
• Patient weighs between 45 and 136 kg (99 and 300 lbs)
• Patient is adequately hydrated

Exclusion Criteria:

• Patient is pregnant, breast-feeding, or expecting to become pregnant during the study
• Patient routinely consumes more than 2 alcoholic drinks per day
• Patient has known or suspected prolonged QT, familial long QT syndrome, previous Torsades de Pointes, Brugada syndrome
• Patient has known bradycardia, advanced AV block, or sick-sinus syndrome, unless controlled by a pacemaker
• Patient has severe aortic stenosis
• Patient has atrial flutter
• Patient has Class IV congestive heart failure (CHF)
• Patient has had a myocardial infarction (MI) or acute coronary syndrome (ACS)
• Patient has had cardiac surgery within 30 days
• Patient has known atrial thrombus
• Patient has reversible causes of Atrial Fibrillation
• Patient has failed electrical cardioversion during current episode of Atrial Fibrillation
• Patient has uncorrected electrolyte imbalance
• Patient has clinical evidence of digoxin toxicity
• Patient has received certain antiarrhythmic drugs or intravenous amiodarone within 7 days
• Patient is known to be HIV positive
• Patient has a history of cancer within the past 5 years, except for certain skin or cervical cancers